http://source.valvesoftware.com/SourceBrochure.pdf
I don't see how Portal 2 being available for Mac directly means that the Source Engine itself can natively support OS X. A direct example is the presence of Orange Box on PS3. Those Source Engine games were ported to the PS3 by EA and not Valve. The marketing material to license the Source Engine states that the Source Engine can only output code to target the PC and XBox 360 platforms, not the PS3. So individual Source Engine games being ported to OS X does not directly mean that the Source Engine itself can support OS X.
Of course, ideally I hope that the Source Engine can natively support OS X just like it does PC and XBox 360, but we'll have to see if they update their marketing materials to reflect that.
On the graphics card issue, it's a non-issue in Source Engine games. The Source Engine is very flexible and even has fallbacks to DirectX 6. Currently, the most advanced code path in the Source Engine is DirectX 9 with no DirectX 10 codepath. Valve games should play just fine on any Intel Mac with a discrete GPU. Of course, older models will need lower settings.
Where the lack of top-end GPU power is questionable is whether it'll discourage the porting of other AAA games to Mac even if Steam for Mac should be a boost for Mac gaming. Still, while Mac's don't usually have the fastest graphics cards in their generation, they are still decent and would fit the minimum or recommended requirements of most games.
And as others have said, the problem is as much driver support as hardware support. Windows drivers are more optimized than OS X drivers so performance is being left on the table. Quicker responses with driver fixes and optimization for new games like on Windows is also required to encourage developer support.